This diploma thesis deals with the experimental study of the tribological behaviour of DLC coatings under boundary-to-mixed lubrication conditions, with a focus on the effect of fatty acids used as additives in the PEG400 base lubricant. The aim of the thesis was to clarify the influence of the DLC coating type, lubricant composition, operating temperature and surface topography on the coefficient of friction, wear, changes in surface topography and lubricant film formation. The experimental part was carried out using RTEC SMT-5000, PCS MTM and Bruker UMT tribological devices in pure reciprocating sliding and rolling-sliding contact configurations. The work included measurement of the coefficient of friction, evaluation of surface topography changes and wear using optical profilometry, and optical observation of lubricant film formation during tribological testing.
The results showed that the tribological behaviour of DLC coatings is not determined only by the coating type or lubricant, but mainly by their combination with contact kinematics and operating conditions. The addition of fatty acids to PEG400 did not result in a universal improvement of tribological behaviour; however, a more pronounced effect was observed in rolling-sliding contact. For the 100Cr6/a-C:H combination, mixtures containing fatty acids reduced the coefficient of friction by up to 60% compared with pure PEG400. Increasing the temperature to 60 °C was not shown to be the primary factor leading to a further significant reduction in friction. In terms of coating type, hydrogenated a-C:H proved to be the more stable system. In pure sliding, the a-C:H/a-C:H combination achieved approximately two orders of magnitude lower wear than ta-C/ta-C, reaching the level of 10⁻⁹ mm³/N·m. Optical observation of the contact confirmed differences in the ability of individual lubricants to form a lubricant film and supplemented the tribological data with direct information from the contact area. The main contribution of the thesis lies in the comprehensive comparison of identical material and lubricant systems under different kinematic regimes and in linking friction, wear, changes in surface topography and optical observation of lubricant film formation.